1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device. The present invention also relates to a method of manufacturing a non-volatile semiconductor memory device.
2. Description of the Related Art
In conventional stacked non-volatile semiconductor memory devices, an insulating film (to be referred to as a second gate insulating film hereinafter) for insulating a floating gate and a control gate from each other is a single layer of silicon oxide. Recently, semiconductor devices tend to be miniaturized more and more, and in this situation the thickness of the second gate insulating film is required to further decrease.
To meet this requirement, a second gate insulating film, a so-called "ONO film", having a stacked structure consisting of a silicon oxide film/a silicon nitride film/a silicon oxide film, is used instead of a single film of silicon oxide. The ONO film has a good withstand voltage characteristic even with a relatively thin film thickness. Also, a retention characteristic of the memory cell having the ONO film is improved.
IEEE Electron Device Vol. 39, No. 2, 1992, p. 283-291 has proposed a structure, a so-called "NO film", consisting of a silicon nitride film/a silicon oxide film, in order to further decrease the thickness of the second gate insulating film.
A non-volatile semiconductor memory device using the second gate insulating film with the above stacked structure is manufactured as follows. As illustrated in FIG. 1, on the surface of a tunnel oxide film 12 formed on the surface of a silicon substrate 11, a first polysilicon film 13 as a floating gate, an ONO film 14, a second polysilicon film 15 as a control gate, and a tungsten silicide film 16 are stacked in this order. The resultant stacked structure is patterned into a memory cell 10 in accordance with a conventional method. Thereafter, thermal oxidation is performed to form side oxide layers 17 on the side surfaces of the first and second polysilicon films 13 and 15 of the memory cell 10.
In the memory cell 10 using the ONO film 14, a silicon oxide film 14a is in direct contact with the first and second polysilicon films 13 and 15. In the method described above, therefore, when the side oxide layers 17 are formed by thermal oxidation, the silicon oxide films 14a grow inward along the interface between the silicon oxide films 14a and polysilicon films 13 and 15, at the end portion of the ONO film 14, thereby forming so-called gate birds' beaks 18, as in FIG. 1. This gate birds' beaks 18 increase the effective thickness of the ONO film 14 and decrease the coupling ratio of the memory cell 10. Accordingly, to ensure the effective applied voltage to the memory cell 10, it is necessary to raise the applied voltage to the control gate. Such a raise in the applied voltage, results in a heavy load on a voltage generating circuit and less reliability of the second gate insulating film. This problem is similarly found in the case of the NO film.
One conventional approach to eliminate this problem is, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-57279, to form an oxidation-resistant film, such as a silicon nitride film, in place of the side oxide film 17, thereby preventing the formation of the gate birds' beak 18. Unfortunately, the formation of this kind of a film is impractical because, it makes the manufacturing process extremely complicated.